(1) Field of the Invention
The present invention relates to an optical receiver and an optical transmission apparatus. The invention particularly relates to an optical receiver and an optical transmission apparatus suitable for use in a wavelength division multiplexing (WDM) optical transmission system.
(2) Description of the Related Art
FIG. 16 depicts a previous WDM transmission system. As shown in FIG. 16, light signals output from more than one optical sender 100 are multiplexed by a multiplexer 200 and then amplified by an optical amplifier 300 before being sent out to a transmission path 400. On the receiver end, the incoming WDM signals are amplified by an optical amplifier 500 and split (demultiplexed) by a demultiplexer 600 among separate optical receivers 700 for each wavelength.
Here, the following patent document 1 shows an example in FIG. 16 where an optical filter array serves as the aforementioned optical demultiplexer 600. Document 1 proposes a method for controlling wavelengths in an optical transmission system and also an optical receive system following the wavelength control method. According to the method, an optical tree coupler or the like branches (splits) WDM signals (λ1 to λn, and λabs) into n+1 light signals, and then, a filter array having as many inputs and outputs as the number of the multiplexed wavelengths plus 1 transmits all the required wavelengths, thereby rendering unnecessary wavelengths attenuated. This filter array is controlled as follows: a pilot light signal, prepared as an exclusive wavelength of λabs, is detected by an photoreceptor device independently prepared, and on the basis of the detected pilot signal, a control circuit drives a Peltier device provided for the filter array, thereby controlling the filter array in a collective way.
The following patent document 2 proposes an art (wavelength selector) for use on the receiver end. This technique uses a single optical tunable filter (OTF) to selectively receive a light signal at a desired wavelength, out of the incoming WDM signals. More precisely, on the sender end, ID signals having different frequencies are superimposed, one on each of the light signals transmitted on separate optical channels; on the receiver end, the OTF transmits a light signal at a desired wavelength, and the filter characteristic of the OTF is controlled in such a manner that the amplitude of the ID signal superimposed on the transmitted light signal becomes the maximum. It is whereby possible to select a light signal at a desired wavelength with crosstalk being minimized, even if channel spacing or optical power levels of the WDM signals are varied.
[Patent document 1]
Japanese Patent Application Laid Open No. HEI 08-237203
[Patent document 2]
Japanese Patent Application Laid Open No. HEI 11-122221
The previous techniques have the following problems. In the techniques shown in FIG. 16 and patent document 1, there is need for preparing multiplexers and demultiplexers equal in number to signals multiplexed in the WDM system, or an optical tree coupler or an optical filter array having as many ports as the number of the signals multiplexed in the WDM system. For instance, let us consider a case where a minimum number of optical senders and optical receivers are prepared for a small number of channels due to low network load (at the time of initial installation, for example). In this case, in anticipation of future demands (increase in number of channels), the previous construction needs to prepare a greater number of multiplexers and demultiplexer than is actually required at the initial installation of the system, or an optical tree coupler and an optical filter array with a greater number of ports than is necessary at the time of installation.
Further, when an optical tree coupler is used as disclosed in patent document 1, the greater the number of resultant signals the WDM signals are split into, the greater become losses being caused, so that the intensity (level) of light signals entering the optical receivers is lowered. Furthermore, for the purpose of collectively controlling the optical filter array, it is required to prepare a dedicated pilot light sender and a dedicated receiver for monitoring the pilot light, and at least one of the channels in the band is whereby occupied so that a total transmission amount is resultantly reduced.
In WDM systems, it is required that a maximum number of optical channels (commonly, 16 or more) are packed in a limited wavelength bandwidth as densely as possible (spacing between adjacent channels should be about 0.4 nm to 1.6 nm), and recently, DWDM (Dense WDM) systems have increasingly been introduced in backbone networks on a commercial basis. Such DWDM systems raise the necessity of wavelength control such that light signals are kept within a required band. The wavelength control function has a drawback that temperature of a light-emitting device and an operation current must be accurately controlled, thereby increasing manufacturing cost.
Therefore, a reasonable CWDM (Coarse WDM) system has recently been developed which requires no such accurate wavelength control. In the CWDM system, standardization of a relatively wide channel spacing of about 20 nm has recently been promoted so that, even if wavelength variations are caused due to manufacture variations of light-emitting devices, or variations in temperature or driving current, crosstalk into an adjacent channel will be able to be minimized, thereby preventing receive sensitivity being deteriorated.
Generally speaking, in WDM transmission systems, optical amplifiers are disposed on the sender end, at relay points, and on the receiver end, or alternatively, one optical amplifier can be arranged at the most appropriate position alone of these, so as to increase transmission distance. Such an optical amplifier is realized by a rare-earth doped optical fiber, a distributed Raman amplifier, a concentrated Raman amplifier, a semiconductor amplifier, or the like, and all of these generate amplified spontaneous emission (ASE) light. The ASE light, if entering a receiver, acts as noise, and the receive sensitivity of the receiver is whereby deteriorated, so that the effect of increasing transmission distance caused by the optical amplifiers is obstructed.
In the above CWDM system, in particular, wavelength variations are caused due to manufacture variations of light-emitting devices on the sender end and variations in temperature or driving current. Thus, even if such manufacture variations are successfully minimized, temperature-dependent variations in wavelength will still appear. Taking this into consideration, the wavelength bandwidth of each signal passing through the demultiplexer is set as wide as 13 nm to 14 nm. In this case, if an optical amplifier is placed before the demultiplexer, it will cause a great amount of ASE light to enter an optical receiver disposed after the demultiplexer. As a result, the optical amplification effect will be suppressed, or the transmission distance can even be reduced, far from being lengthened, if worsened by where to place the optical amplifier.
Further, according to the art (wavelength selector) disclosed in patent document 2, it is possible to selectively receive a signal at a desired wavelength with crosstalk being minimized, even when channel spacing or optical power levels of the WDM signals are not constant. However, if the wavelength selector is applied to the CWDM system and its transmittable wavelength bandwidth is set as wide as 13 nm to 14 nm as described above, a great amount of ASE light will likewise enter the receiver.
Further, since only a single wavelength can be received at a time in the above art, the transmittable wavelength of the OTF needs to be switched into a time-division system, or wavelength selectors identical in construction need to be prepared, one for each channel multiplexed in the WDM signals. In the former, since the switching rate depends on response characteristics of the OTF, some channels are expected to become unavailable, in high-speed optical transmission systems, according to the switching rate. On the other hand, the latter has a disadvantage of great increase in system size and cost.